The present invention relates to the field of fiber-optic pressure and temperature measurement. It proceeds from a fiber-optic sensor according to the preamble of claims 1 and 12.
In oil production, drill holes have to be monitored with regard to pressure and temperature. The liquid pressures in the drill hole can be up to 100 MPa (1000 bar), and the temperatures can be up to over 200xc2x0 C. Electric sensors such as, for example, piezoelectric resistors, piezoelectric elements, capacitive probes or crystal resonators, or optical pressure sensors such as, for example, Fabry-Perot resonators or elastooptic sensors are frequently used in pressure measurement up to approximately 170xc2x0 C.
A fiber-optic pressure sensor in accordance with the preamble is known from the article by M. G. Xu et al., xe2x80x9cOptical In-Fibre Grating High Pressure Sensorxe2x80x9d, Electronics Letters 29 (4), pages 398-399 (1993). There, fiber Bragg grating sensors are presented for measuring isotropic pressures of liquids. The Bragg grating of a sensor fiber is exposed directly to the all round hydrostatic pressure of a fluid. A substantial disadvantage consists in that the isotropic pressure sensitivity for Bragg gratings in glass fibers is exceptionally low (typically 0.0003 nm/100 kPa specific Bragg wavelength displacement at 1550 nm). In addition, because of the high temperature sensitivity (typically 0.01 nm/xc2x0 C.), it is necessary to compensate temperature effects.
An optical sensor with fiber Bragg gratings for measuring material elongations is disclosed, for example, in U.S. Pat. No. 4,761,073. For the purpose of monitoring body deformations, the sensor fiber is typically fastened on the surface of the body or embedded in the body. It is proposed to eliminate signal interference owing to thermal grating elongations with the aid of superimposed gratings of various reflection wavelengths.
U.S. Pat. No. 5,042,898 exhibits a temperature-stabilized fiber Bragg grating which can be used as wavelength standard to stabilize the emission wavelength of laser diodes, or as a wavelength filter in fiber optic sensors. The fiber is held between two supports of suitable thermal expansion and length such that the thermally induced changes in the Bragg wavelength are compensated.
It is the object of the present invention to specify a fiber Bragg grating pressure sensor which is suitable for measuring differential isotropic pressures in liquids or gases and is distinguished by good measuring sensitivity and a large measuring range. This object is achieved according to the invention by means of the features of claims 1 and 12.
The invention specifies a fiber-optic sensor for differential pressure measurements which comprises a transducer with pressure members for holding two fluids, the transducer being configured for converting the medium pressures into a longitudinal elongation or compression of at least one fiber Bragg grating of a sensor fiber. The transducer therefore exchanges pressure with the two fluids, is deformed by their pressures and transforms the deformation into a change in length of the sensor fiber in the region of a fiber Bragg grating. The deformation of the transducer depends on the absolute pressures and/or directly on the differential pressure.
In first exemplary embodiments, a fiber Bragg grating is held between two pressure members which can be elongated by the pressures of the fluids.
In second exemplary embodiments, a fiber Bragg grating is held between a supporting member fastened on the transducer housing and a pressure member which can be elongated by the pressure difference between the two fluids.
In addition, for the purpose of error compensation, a fiber Bragg grating can be fitted between the pressure members or a pressure member and supporting member such that the measuring signal is oppositely directed and interfering signals are codirectional, and a doubled noise-free difference signal can be formed.
Another exemplary embodiment constitutes a serial, reflexive multiplex arrangement of a plurality of fiber Bragg grating differential pressure sensors with different Bragg wavelengths which are fed via a common broadband light source and detected in a wavelength-selective fashion.
A preferred application of the differential pressure sensor is use in conjunction with a venturi tube for the purpose of determining a flow rate.
Further designs, advantages and applications of the invention follow from the dependent claims and from the description, which now follows, with the aid of the figures.